1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copy machine, a printer, a facsimile machine and a multi-function machine capable of copying, printing, and faxing, and more particularly to an image fixing apparatus which uses an induction heater and is capable of stably controlling a fixing temperature
2. Description of the Background Art
An image fixing apparatus using an electromagnetic-induction heating (IH) method has been known, which aims at saving energy by shortening the startup time of an image formation apparatus, such as a copying machine and a printer. For example, an improved image fixing apparatus was disclosed in Laid-open Japanese Patent Application No. 2005-258383 as Patent Reference 1. According to Patent Reference 1, for example, the image fixing apparatus of an electromagnetic-induction heating method includes a fixing belt, serving as a fixing member, that is installed with tension, and supported by a heating roller and an auxiliary fixing roller an electromagnetic-induction heating unit (IH unit) that is installed countering the heating roller with the fixing belt in-between, and a pressurizing roller that is installed countering the auxiliary fixing roller with the fixing belt in-between.
Further, the IH unit includes a coil unit and a core that is installed countering the coil unit, the coil unit being installed in the width direction (the direction that perpendicularly intersects the conveyance direction of the recording medium). Further, the heating roller includes a magnetic flux shield member which is arranged to make a heating range of the heat member variable at both ends of the core in the width direction. Here, the fixing belt is heated at a position that counters the IH unit. The heated fixing belt provides heat to a toner image on the recording medium conveyed to the position of the auxiliary fixing roller and the pressurizing roller such that the toner image is fixed. Specifically, a high frequency alternating current is provided to the coil unit, which generates a magnetic field around the coil unit, causing an eddy current to arise on the surface of the heating roller. The eddy current in the heating roller generates Joule heat due to the electric resistance of the heating roller. The Joule heat raises the temperature of the fixing belt that is installed around the heating roller. Further, the magnetic flux shield member reduces the heating range of the heat member in order to correspond the heat range to the width range of a small width recording medium after an image fixing operation is continuously performed on the small width recording medium by the image fixing apparatus.